A collection usually contains photosets and it may come in handy when you have a lot of them.
For more details, please see this help article from Yahoo.
Flickr Mobile allows viewing existing collections and their contents, editing a collection's title and description, deleting an existing collection as well as regenerating the collection's icon. (The icon is actually a mosaic composed of various photos in the contained photosets.)
You can add an existing photoset to a collection in the Photosets section, by tapping on the context menu and choosing Add to collection.
In the Activity section, you can see the most recent comments and faves on your photos, photosets and galleries.
Finally, we improved the styling throughout the app, using Google's "cards" UI pattern, as seen in the Google Music app, as well as in Google Play.
Note: This has been published as a staged rollout and it may take a day or so before it becomes available in all geographical regions.